Hydrogen is utilized in a wide variety of industries ranging from aerospace to food production to oil and gas production and refining. Hydrogen is used in these industries as a propellant, an atmosphere, a carrier gas, a diluent gas, a fuel component for combustion reactions, a fuel for fuel cells, as well as a reducing agent in numerous chemical reactions and processes. In addition, hydrogen is being considered as an alternative fuel for power generation because it is renewable, abundant, efficient, and unlike other alternatives, produces zero emissions. While there is wide-spread consumption of hydrogen and great potential for even more, a disadvantage which inhibits further increases in hydrogen consumption is the absence of a hydrogen infrastructure to provide widespread generation, storage and distribution. One way to overcome this difficulty is through distributed generation of hydrogen, such as through the use of fuel reformers to convert a hydrocarbon-based fuel to a hydrogen-rich reformate.
Fuel reforming processes, such as steam reforming, partial oxidation, and autothermal reforming, can be used to convert hydrocarbon fuels such as natural gas, LPG, gasoline, and diesel, into hydrogen-rich reformate at the site where the hydrogen is needed. However, in addition to the desired hydrogen product, fuel reformers typically produce undesirable impurities that reduce the value of the reformate product. For instance, in a conventional steam reforming process, a hydrocarbon feed, such as methane, natural gas, propane, gasoline, naphtha, or diesel, is vaporized, mixed with steam, and passed over a steam reforming catalyst. The majority of the hydrocarbon feed is converted to a mixture of hydrogen and impurities such as carbon monoxide and carbon dioxide. The reformed product gas is typically fed to at least one water-gas shift bed in which the carbon monoxide is reacted with steam to form carbon dioxide and hydrogen. After the shift reaction(s), additional purification steps are required to bring the reformate purity to acceptable levels. These steps can include, but are not limited to, methanation, selective oxidation reactions, passing the product stream through membrane separators, as well as pressure swing and temperature swing absorption processes. While such purification technologies may be known, the added cost and complexity of integrating them with a fuel reformer to produce sufficiently pure hydrogen reformate can render their construction and operation impractical.
In terms of power generation, fuel cells typically employ hydrogen as fuel in catalytic oxidation-reduction reactions to produce electricity. As with most industrial applications utilizing hydrogen, the purity of the hydrogen used in fuel cell systems is critical. Specifically, because power generation in fuel cells is proportional to the consumption rate of the reactants, the efficiencies and costs of fuel cells can be improved through the use of highly pure hydrogen reformate. Moreover, the catalysts employed in many types of fuel cells can be deactivated or permanently impaired by exposure to certain impurities that are commonly found in conventionally reformed fuels. As a result, an improved yet simplified reforming apparatus and process capable of providing a high purity hydrogen reformate that is low in carbon oxides is greatly desired.
Single step reforming (SSR) combines steam methane reforming (SMR), water gas shift, and carbon dioxide (CO2) removal in a single step process for hydrogen generation. The SSR reactor includes a reactor vessel having an inlet for receiving a hydrocarbon fuel and an outlet for delivering a hydrogen-rich reformate. Disposed within the reactor vessel is a catalyst bed that includes a reforming catalyst, a carbon dioxide fixing material and a water gas shift catalyst.
Currently, there are various reactor systems for SSR. The most common reactor system includes a fixed bed reactor system resembling a plug flow reactor for the SMR. In this system, a concurrent reaction using calcium oxide (CaO) converts the CO2 generated via SMR to calcium carbonate (CaCO3). This reaction enhances the SMR by creating a thermodynamic shift (enhancement) of the SMR and generates heat to compensate for the SMR endothermic reaction. The combined effect is that in an SSR reactor, there will be a cumulative effect to increase and maintain the SMR reaction at very high rates. In addition, the SSR system yields a very high concentration hydrogen stream.
However, the reaction of CaO with CO2 is a gas to solid reaction whereby the sorbent CaO particles are continuously depleted in the typical plug flow fixed bed reactor. Consequently, shortly after the start of the SMR reaction, the lower sections of the reactor bed are saturated with CO2 and therefore, the enhancement of SSR will diminish as the CaO to CaCO3 saturation front propagates axially across the bed. Eventually, the lower sections of the fixed bed reactor will reduce to a SMR system progressively operating at sub-optimal conditions and lower reaction temperatures.
In addition, the use of combined CaO and SMR catalysts in a fixed bed dictates a semi-batch operation whereby the spent sorbent particles have to be regenerated from CaCO3 to CaO. This process involves heating the sorbent in place by flowing superheated steam and/or exhaust combustion gas at a temperature above 750° C. This cyclical thermal re-processing and semi-batch operation may yield a sub-optimal net thermal energy efficiency of the overall system, may require a larger reactor system, may complicate operations, and may cause thermal degradation of the sorbent particles which cannot be easily accessed for replenishment. The present invention addresses these issues associated with the reactor system for SSR.